Recovering hexavalent chromium for reuse

ABSTRACT

In recovering hexavalent chromium from chrome plated steel strip which is wetted with plating solution as a result of dragout from the plating process, an aqueous solution containing hexavalent chromium in relatively low concentration is concentrated in a process comprising treating in a first cation exchange zone, treating in an anion exchange zone, regenerating the anion exchange zone and treating the effluent of regeneration in a cation exchange zone different from the first cation exchange zone. The concentrated solution is passed to a storage zone and recirculation is carried out between the storage zone and the plating zone and solution is passed from the storage zone to an evaporation zone and more concentrated solution is passed from the evaporation zone to the storage zone. In regenerating the anion exchange zone, only a portion of the hexavalent chromium is removed.

United States Patent [191 Smith et a1.

[11] 3,903,237 1 Sept. 2, 1975 RECOVERING HEXAVALENT CHROMIUM FOR REUSE [73] Assignee: National Steel Corporation,

Pittsburgh, Pa.

[22] Filed: June 4, 1973 [21] Appl. No.: 366,966

[52] Cl. 423/54; 423/6585; 204/51; 210/32; 210/37; 210/38; 75/101 BE [51] Int. Cl COlg 37/12; COlg 37/14 [58] Field of Search 423/54, 658.5; 204/51; 210/37, 32, 38; 75/101 BE [56] References Cited UNITED STATES PATENTS 2,733,204 l/l956 Costa 423/54 X 3,223,620 12/1967 Oberhofer..... 210/37 X 3,658,470 4/1972 Zievers ct al. 210/37 3,661,732 5/1972 Withrow 204/51 3,681,210 8/1972 Zievers et al. 204/51 OTHER PUBLICATIONS Culotta et al., Plating, March 1970, pp. 251-255.

Pautson et al., Plating, September 1005-1009.

Primary Examiner-I-Ierbert T. Carter Attorney, Agent, or F irm--Shanley, ONeil and Baker 5 7 ABSTRACT In recovering hexavalent chromium from chrome plated steel strip which is wetted with plating solution as a result of dragout from the plating process, an aqueous solution containing hexavalent chromium in relatively low concentration is concentrated in a process comprising treating in a first cation exchange zone, treating in an anion exchange zone, regenerating the anion exchange zone and treating the effluent of regeneration in a cation exchange zone different from the first cation exchange zone. The concentrated solution is passed to a storage zone and recirculation is carried out between the storage zone and the plating zone and solution is passed from the storage zone to an evaporation zone and more concentrated solution is passed from the evaporation zone to the storage zone. In regenerating the anion exchange zone, only a portion of the hexavalent chromium is removed.

2 Claims, 2 Drawing Figures RECOVERING HEXAVALENT CHROMIUM FOR REUSE BACKGROUND OF THE INVENTION This invention relates to recovering hexavalent chromium and increasing the concentration of the same in aqueous solution. Such allows practical use for plating of chromium values which would otherwise be lost.

Methods have been disclosed for recovery of hexavalent chromium involving washing plated articles wetted with chromium-containing solution, recovering an aqueous solution containing hexavalent chromium in relatively high concentration, recovering an aqueous solution containing hexavalent chromium in relatively low concentration, concentrating low concentration solution by ion exchange, and concentrating by evaporation. In this regard, see Paulson et al., Plating, pages 1005-1009, September, 1953; Culotta et al, Plating, pages 251-255, March, 1970; Zievers et al. US. Pat. No. 3,658,470.

It is an object of embodiments of this invention to provide a process of the above type having novel features which each independently contribute to cost savings and increased efficiency.

It is a further object of embodiments of this invention to provide a process which is particularly adapted to recovering hexavalent chromium from plating solution wetting chrome plated steel strip as a result of dragout from the plating process.

It is another object of embodiments of this invention to provide a process utilizing ion exchange resin wherein cost of such resin is minimized.

Still another object of embodiments of this invention is to provide a particular process involving evaporation whereby concentration is more accurately controlled and temperature control difficulties are eliminated.

These objects and others will be evident from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B constitute a flowsheet depicting a process for chrome plating steel strip and for recovering hexavalent chromium from the solution wetting the plated strip for reuse and illustrate the inventive concepts herein.

DETAILED DESCRIPTION With continuing reference to FIGS. 1A and 1B of the drawings, steel strip having been preliminarily treated, for example in electrolytic cleaning and pickling steps, follows a travel path and passes successively through chrome plating tanks 12, 14, 16 and 18 which define a plating zone. In each of the plating tanks 12, 14 and 16, the strip follows the travel path so as to enter a plating tank in a substantially horizontal plane, then passes over a contact roll and advances downwardly, then passes under a sink roll and advances upwardly, then passes over another contact roll and leaves the tank in a substantially horizontal plane. In the plating tank 18, the strip follows the travel path so as to enter the tank in a substantially horizontal plane, then passes over a contact roll and advances downwardly, then passes under a sink roll and advances upwardly, then passes over a deflector roll and leaves the tank in a substantially horizontal plane. In FIG. 1A the contact rolls at the strip entrance sides of tanks l2, 14, 16 and 18 are respectively denoted 12a, 14a, 16a and 18a; the sink rolls are respectively denoted 12b, 14b, 16b and 18b; the contact rolls at the exit sides of the tanks 12, 14 and 16 are respectively denoted 12c, 14c and 160; and the deflector roll at the exit side of tank 18 is denoted 18c. The contact rolls 12a, 14a, 16a, 18a, 12c, 14c and 16c perform a deflecting function and also render the strip cathodic. At the entrance end of each of the tanks 12, l4, l6 and 18, snubber rolls respectively denoted 12d, 14d, 16d and 18d force the strip against adjacent contact rolls to provide good contact.

In the first three tanks, that is tanks 12, 14 and 16, there are two sets of anodic grids which are vertically oriented with one of the sets being between the contact roll at the entrance end of a tank and a sink roll and the other set being between a sink roll and the contact roll at the exit end of a tank so that the strip passes through one set of grids on its downpass through a tank and through the other set on its up pass. The grids for the downpass are respectively denoted 12e, 142 and 16a, and the grids for the up pass are respectively denoted 12f, 14f and 16f. In the fourth tank, tank 18, there is only one set of vertically oriented grids, and this set is between the contact roll at the strip entrance end of the tank and the sink roll so that the strip passes through this set on the downpass and does not pass between any grids on the up pass; this set of grids is denoted l8e.

The electrolyte in each of the tanks 12, 14, 16 and 18 is the same, namely an aqueous solution of chromic acid, and is depicted as a body of liquid in each tank extending to a level above the anodic grids. The chemis try of chromic acid is quite complicated and what exactly is pesent is not known. However, at the pH of less than 1 which is characteristic of the electrolyte utilized, the chromic acid is mostly in the form of dichromic acid (H- Cr O The concentration in the electrolyte of the chromic acid expressed as grams of CrO per liter is in the range of to 250. Small amounts of silicofluorides (magnesium, sodium or potassium) and sulfate in the form of sulfuric acid are typical additives.

Prewet sprays 19 are present at the entrance end of tank 12 upstream of contact roll 12a to spray plater solution, that is electrolyte, at each side of the strip before it reaches the contact roll. The reason for this is to put a good conducting medium on the strip so that it will make good contact with the contact roll thereby negating arcing between the contact roll and the strip.

The strip having been chrome plated exits from tank 18 and follows travel path 10 through a tank 20. The strip enters the tank 20 in a horizontal plane, passes over a deflector roll 20a, then turns downwardly traveling vertically, then passes under a sink roll 20b, then advances vertically upwardly and passes over a deflector roll 20c and then advances horizontally out of the tank. The tank 20 contains the same solution as each of the plating tanks l2, 14, 16 and 18 but no electrolytic action is imparted during the passage of the strip through this solution. Passage of the strip through the tank 20 has the effect of removing objectionable oxide coating from the strip.

At the exit side of tank 20 downstream of deflector roll 200 is a pair of wringer rolls 22 between which the strip passes. The rolls 22 wipe plater solution from the strip to minimize dragout of electrolyte from the plating process.

The tanks 12, 14, 16, 18 and 20 are continuously supplied with electrolyte, that is plater solution, respectively through valved lines 12g, 14g, 16g, 18g and 20g.

These lines are supplied with plating solution by a header 24. Header 24 also communicates with valved lines 26a and 26b which in turn communicate with sprays 19. The header 24 communicates with and is fed byline 28 which in turn communicates with and is fed byvalved line 30 which in turn communicates with plater solution storage tank 32 which defines a plater solution storage zone. A pump 34 is provided in line 28 to continuously move the plater solution from tank 32 via lines 30 and 28 and into header 24 and from there via lines 26a and 26b and 12g, 14g, 16g, 18g and 20g into tanks l2, 14, 16, 18 and 20. A valved line 36 communicates with line 30 so that plater solution can be drained from the system. Overflow outlets are provided in each of the tanks 12, 14, 16, 18 and 20, and plater solution continuously overflows through these outlets through the respective lines 12h, 14h, 16/2, 18/1 and 2011. These overflow lines communicate at their downstream ends into a main exit line 38 which communicates with another line 40 which in turn communicates with the plater storage tank 32. Thus, there is provided between tank 32 and between tanks 12, 14, 16, 18 and 20 a recirculation loop wherebyrecirculation is continuously carried out between the storage Zone defined by tank 32 and the plating zone defined by tanks 12, 14, 16 and contains a pump 48 whereby plater solution is recirculated through this loop. Water is passed through heat exchanger 42 countercurrent to the plater solution as indicated by lines 50a and 50b to remove heat from the plater solution (thatis, from the plating solution storage zone defined by tank 32). This is done because there is a heat buildup in the plating portion of the system due to the grid current. In other words, the heat gained because of the current supplied by the grids exceeds the heat lost through the tanks and pipes of the system. Inasmuch as the best cathode efficiency is achieved utilizing a plater solution having a tempera ture of 105F. to 125F., control of heat buildup is desirable.

The chrome plated strip having had undesirable oxides removed in tank 20 and having passed through wringer rolls 22 still is wetted with a significant amount of plating solution comprising aqueous chromic acid solution. So as to recover the hexavalent chromium from such solution, the strip is advanced along travel path through a first rinse tank 52 and a second rinse tank 54. The strip is then passed through a chemical treatment tank 56. The strip is then passed through a third rinse tank 58 to recover hexavalent chromium from solution wetting the strip.

The tank 52 contains a strip entrance end deflector roll 52a, a sink roll 52b and a strip exit end. deflector roll 52c and the strip following travel path 10 turns downwardly over roll 52a and moves vertically downwardly turning under roll 52b and then advances vertically upwardly over roll 52c whereupon it leaves tank tically downwardly, then turns under roll 54b, and then advances vertically upwardly and turns over roll 54c and moves out of tank 54.

The tank 56 contains a strip entrance end contact roll 56a, a sink roll 56b and a strip exit end contact roll 56c. The strip exiting from tank 54 passes over roll 56a and then passes vertically downwardly, then passes under roll 56b, then advances vertically upwardly and passes over roll 56c exiting from tank 56 horizontally.

The tank 58 contains a strip entrance end deflector roll 58a, a sink roll 58b and a strip exit end deflector roll 58c. The strip exiting from tank 56 passes into tank 58 and over roll 58a, then advances vertically downwardly, then passes under roll 58b, then passes vertically upwardly, then passes over roll 58c and exits horizontally from tank 58.

In tanks 52 and 54, the strip is subjected to rinsing to wash off plater solution on the strip by dragout from the plater tanks 12, 14, 16 and 18 and from tank 20. In the chemical treatment tank 56, the strip is subjected to electrolytic treatment to provide a hexavalent chrome oxide coating. In tank 58, washing is provided to remove solution on the strip by dragout from the chem treat tank 56. i

In the chemical treatment tank, the electrolyte is preferably aqueous chromic acid solution containing 35 grams per liter of chromic acid expressed as CrO Electrolytic action is supplied in this tank, and it operates and is constructed generally as one of the plater tanks l2, l4, 16 or 18 with vertically oriented anodic grids 56k: and 56f on either side of the strip respectively on a downpass and on an up pass and contact rolls 56a and 56c rendering the strip cathodic and snubber roll 56d forcing the strip against contact roll 56a at the inlet to the tank. As with the plater tanks, a dc current is supplied. The electrolyte is initially supplied into the tank 56 from a chemical treatment storage tank, not depicted, via valved line 60. Solution is recirculated between the chemical treatment tank and the chemical treatment storage tank during the progress of the process with electrolyte flowing from the storage tank into tank 56 via line 60 and returning to the chemical treatment tank by overflow gravity feed via line 62. The recirculation loop includes a heat exchanger, not depicted, where recirculating solution passes in indirect heat exchange with hot water to heat the recirculating liquid and control the temperature of solution in the storage tank, for example to 105F. A pair of wringer rolls 64 on either side of the strip at the exit end of tank 56 wipe electrolyte from the strip and minimize the amount of the electrolyte leaving the tank 56 by dragout on the strip.

The tanks 52, 54 and 58 make up an interconnected washing system with washing solution passing counterflow to the strip wherein the strip is washed in a first washing step to recover an aqueous solution comprising a relatively high concentration of hexavalent chromium in the form of anions.

On startup, rinse solution, preferably demineralized water is introduced into tank 52 via valved line 66. Also, rinse solution is initially introduced into tank 54 through valved line 68 and into tank 58 through a valved line 70. Lines 68 and 70 are supplied through header 72 which is supplied from a rinse water storage tank, not depicted. v

During the process, demineralized water is added continuously into tank 58 through spray nozzles 74 which are supplied by line 76 containing valve 78. The spray nozzles 74 are on either side of the strip in tank 58 on its up pass just prior to its passage over roll 58c. Flow through the spray nozzles 74 is preferably controlled to maintain a CrO concentration in tank 54 equal to the CrO concentration in tank 56 by manually setting the flow through the nozzles and controlling valve 78 in response to periodic chemical tests carried out on tank 54 solution. These sprays aid in washing off residual electrolyte, that is aqueous hexavalent chromium containing solution, on the strip by dragout from tank 56. Washing action is also provided by a bath 71 in the bottom of tank 58 through which the strip passes in its downpass and in its up pass through tank 58. The demineralized water introduced through the sprays 74 and solution washed off of the strip by these sprays falls by gravity into the lower portion of tank 58 and becomes part of the bath 71. Solution continuously overflows through an overflow outlet in tank 58 providing a constant level in that tank.

The liquid leaving tank 58 through its overflow outlet enters a line 80 and flows therethrough by gravity into a collection tank 82. The level of liquid in tank 82 is sensed by level sensor 84. A level controller 86 operates in response to the level sensed, and a pump 88 operating in response to a set point on the controller 86 operates to pump solution from tank 82 via a'line 90 into tank 54.

The tank 54 has an overflow outlet, and this acting in concert with addition of solution through line 90 provides a substantially constant level bath in tank 54. The strippassing through tank 54 is washed by the bath on its downpass and up pass through tank 54 whereby aqueous hexavalent chromium containing solution on the strip by dragout from the plating tanks and tank 20 is rinsed therefrom.

Solution passes out of the overflow outlet in tank 54 and passes via a line 92 into tank 52. A substantially constant level of rinse solution is maintained in the lower portion of tank 52 as a result of an overflow outlet in that tank. The bath of rinse solution in tank 52 provides Washing of the strip on its downpass from roll 52a to roll 52b and on its up pass from roll 52b to roll 52c whereby aqueous hexavalent chromium containing solution on the strip by dragout from the plating tanks and tank 20 is washed therefrom.

Solution overflows through the outlet in tank 52, flows by gravity through a line 94 and then into main line 38 and through line 40 into plater storage tank 32.

The concentration of chromic acid expressed as CrO in the baths in each of the tanks 52, 54 and 58 is as follows: in the bath in tank 52, 85 to 100 grams per liter; in the bath in tank 54, 30 to 40 grams per liter, preferably 35; in the bath 71 in tank 58, 5 to grams per liter, preferably 10. These concentrations are controlled by control of valve 78.

Following its exit from tank 58, the chrome plated strip still wetted with some aqueous hexavalent chromium containing solution, for example one-tenth of the amount removed in the prior washing in tanks 52, 54 and 58 continues along travel path 10 through a spray washer 96 depicted in FIG. 18 wherein the strip is washed in a second washing step to recover an aqueous solution comprising cations (for example, iron and trivalent chromium ions) and a relatively low concentration of hexavalent chromium in the form of anions,

The spray washer comprises four open top compartments 98, 100, 102 and 104 in series along its length with compartment 98 at its strip entrance end, followed by compartment 100, followed by compartment 102 and finally followed by compartment 104 at its strip exit end. A wall 106 separates compartments 98 and 100. A wall 108 separates compartments 102 and 100. A wall 110 separates compartments 104 and 102. The wall 110 has a greater vertical dimension than the wall 108 and the wall 108 has a greater vertical dimension than the wall 106. The strip advancing in its travel path 10 through the spray washer 96 passes first over compartment 98, then over compartment 100, then over compartment 102 and then over compartment 104. The strip path over the compartments is a straight line path and is horizontally oriented. There are five sets of wringer rolls that guide the strip over the compartments in the horizontal path. These sets of wringer rolls are respectively denoted 98a, a, l02a, 10422 with set 980 being at the strip entrance end of the washer over compartment 98, set 100a being over compartment 100 near wall 106, set 102a being over compartment 102 near wall 108, set 104a being over compartment 104 near wall and 1041? being near the strip exit end of the washer 96.

Over each of the compartments 98, 100, 102, 104 there are two spray nozzles, one below the strip and one above the strip so that the strip is sprayed from above and below. The spray nozzles above tank 98 are denoted 98c; those above compartment 100 are de noted 1000; those above compartment 102 are denoted 1020 and those above compartment 104 are denoted 104C. Spray nozzles 1046 are supplied with demineralized water by valved line 112. The water from the spray nozzles 104C washes residual solution containing hexavalent chromium from the strip, and the resultant solu tion falls by gravity into compartment 104 where it builds up to the level of the top of wall 110 and then continuously cascades from compartment 104 into compartment 102. Solution is pumped from the bottom of compartment 102 through a valved line 114 by a pump 116 to the spray nozzles 102c. The solution from the nozzles 102 washes residual solution containing hexavalent chromium from the strip and resultant solution falls into compartment 102, The solution in compartment 102 accumulates to the level of the top of wall 108 and then continuously cascades over this wall into compartment 100. Solution from the bottom of compartment 100 flows through a valved line 118 and is pumped by a pump 120 to spray nozzles 100C. Solution from the spray nozzles 100C washes residual solution containing hexavalent chromium from the strip and the resultant solution falls into compartment 100. Solution accumulates in compartment 100 to fill that compartment and thereafter cascades over wall 106 into compartment 98. Solution flows from the bottom of compartment 98 through a valved line 122 and is pumped by a pump 124 to spray nozzles 98c. Solution from the spray nozzles 98c washes residual solution containing hexavalent chromium from the strip and the resultant liquid falls into compartment 98. Solution overflows from compartment 98 into line 126.

The spray washer 96 constitutes a counterflow spray washing system whereby solution on the strip containing hexavalent chromium is washed therefrom and a solution comprising cations (for example, iron and trivalent chromium ions) and a relatively low concentration of hexavalent chromium, that is, for example, from O.l to 0.3 grams per liter of chromic acidexpressed as CrO overflows into line 126. When the strip leaves washer 96, it contains only trace or no amounts of chromic acid wetting'it.

The strip exiting from the spray washer is dried and coiled.

Merging with line 126 is a line 128 containing a metering pump 130. The upstream end of line 128 com-. municates with a chrome waste storage tank, not shown in the drawings. Various of the tanks can be emptied into the chrome waste storage tank, for example on shut down, to capture chrome laden water which can be metered by pump 130 into line 126 for chrome recovery.

Solution from line 126 enters a collection tank 132 having an exit line 134.

Valved line 133 is provided communicating with tank 132 for the passage thereto of chrome laden water from sources other than compartment 98 and line 128; for example, chrome laden water from washing strip exiting from a chemical treatment step in an electrotinplating line can be passed into tank 132 through line 133.

The valved lines 128 and 133 constitute means for admixing aqueous solution recovered from the second washing step with hexavalent chromium containing solution from other sources to provide in tank 132 an admixture which is an aqueous solution comprising cations and a relatively low concentration of hexavalent chromium in the form of anions.

Solution is pumped from tank 132 by a pump 136 in line 134 in response to a level control 138 operating a valve 140 in line 134. The pump 136 runs continuously and the level control operates to close the valve more if the level in the tank drops. This setup operates to provide continuous flow through line 134.

The solution from ,line 134 flows through heat exchanger 142 where it is cooled against the countercurrent flow of cold water as denoted by arrows 142a and 14212. Cooling is to a temperature suitable for treatment of the solution in the ion exchange system is described below.

Solution leaves the heat exchanger 142 via a line 144 and passes through a filter 146 which removes particles of solid material. The filtered solution flows through line 148 into an ion exchange system which is described below.

The ion exchange system comprises three sets of ion exchangers, each set consisting first of a cation e'xchanger defining a first cation exchange zone and then of an anion exchanger defining an anion exchange zone. The cation exchangers are denoted 150a, 15012 and 150C. The anion exchanger associated with cation exchanger 150a is denoted 152a; the anion exchanger associated with cation exchanger 1501) is denoted 152b; and the anion exchanger associated with cation exchanger 1506 is denoted 152C. Valved lines 154a, l54b and 154c respectively communicate between line 148 and cation exchangers 150a, l50b and 1500. Line 156a communicates between cation exchanger 150a and anion exchanger 152a; line 156b communicates between cation exchanger 150b and anion exchanger 152b; and line 1560 communicates between cation exchanger 150c and anion exchanger 152C. The lines 156a, l56b and 1560 each contain valves. Three setsof the ion exchangers are provided so that two sets can be on line while the exchangers of the other set are being regenerated.

Resins for use in the cation exchangers 150a, 15012 and 150C are of the strong acid type and are in the hydrogen form. They contain sulfonic acid functional groups and are prepared, for example, by the nuclear sulfonation of styrenedivinylbenzene. These resins are not as highly oxidative resistant as the resins utilized in the cation exchanger for treating regeneration effluent which is described later. Suitable cation exchange resins are, for example, Amberlite IR-l20 or Amberlite IR-l20 Plus manufactured by Rohm and Haas Company. Amberlite Plus is described by the manufacturer as a premium-quality resin; the manufacturer has measured the oxidative resistance of premium resins in terms of moisture content increase upon immersion of the resins in 3% hydrogen peroxide solution for 72 hours and has found a moisture content increase from 46% before immersion to a moisture content of 76% at the end of the immersion (72-hour) period.

The ion exchange resin for use in the anion exchangers is a strong base anion exchange resin of the quaternary ammonium type. It is utilized in the hydroxide form. A suitable resin is Amberlite IRA-900 which is supplied in the chloride form and is converted to the hydroxide form for use; it is available from Rohm and Haas Company.

Solution flows from line 148 into whichever two of the cation exchangers a, 150b and 1500 are on stream. The cation exchangers take out cations, including trivalent chromium and iron ions and replace them with hydrogen ions. Effluent from a cation exchanger is aqueous solution containing hexavalent chromium in the form of anions, other anions and hydrogen ions and flows into an associated anion exchanger 152a, l52b or 152C. The anion exchangers take out hexavalent chromium in the form of anions and other anions and replace these with hydroxyl ions. Effluent from the anion exchangers is demineralized water. Valved exit lines for flow of effluent from anion exchangers 152a, l52b and 152C are respectively denoted 158d, 15% and 1580.

The lines 158a, l58b and 1586' each communicate with a main line 160 whereby demineralized water effluent flows into a demineralized water storage tank 162. Included in the line 160 is a device 164 for deaerating the demineralized water. The storage, tank 162 has a valved exit line 166 for flow of water to lines 1 12 (see FIG. 1B) and 76 (see FIG. 1A). A pump 168 is provided in line 166 to pump the water through line When a set of ion exchange units is to be regenerated, it is isolated from flow from line 148 and from line 160 by closing the appropriate valves, and another set of ion exchange units is brought into communication with line 148 and line 160.

Regeneration of the cation exchange unit in a set of units is carried out, for example as suggested by the manufacturer of the particular resin utilized, and the particular method of regeneration forms no part of the present invention. Regeneration of a cation exchange unit can be carried out for example, by backwashing, introducing 10% sulfuric acid as a regenerating agent and rinsing.

Regeneration of an anion exchange unit involves a regeneration cycle comprising backwashing, removal of residual backwash water, introduction of regenerating agent; removal of resulting solution, introduction of rinse water and removal of resulting solution, introduc.

tion of water, and recirculation. For simplification purposes some valves associated with these steps are not depicted. For further simplification, the state of each valve in the system at each point in time during the cycle will not be described; the conduits described as functioning at a particular point in time are open or have valves positioned as otherwise described and the other conduits in the regeneration system at that point in time are closed otherwise stated. The regeneration is carried out to produce an aqueous effluent comprising cations (from the regenerating agent) and a concentration of hexavalent chromium in the form of anions substantially higher than the relatively low concentration leaving compartment 98 or in tank 132. Regeneration of an anion exchange unit is described in detail below.

Backwashing of the anion exchange column is carried out to fluff up the resin and remove extraneous solids and resin fines from the resin bed to permit good contact between the resin and regenerating agent so that regeneration is carried out efficiently. Backwash water having cations removed therefrom, for example by passage through the cation exchanger associated with the anion exchanger being regenerated, is introduced into the bottom of the anion exchange column (the flows other than backwash flows through the various ion exchange columns herein are from the top as is conventional; however, the direction offlow constitutes no part of the present invention). Piping from the introduction of backwash water is represented by valved line 169a, and piping for outlet of backwash water is represented by valved line 169b, both depicted as associated with column 1526. Corresponding pipiing is provided for columns 152a and 15212 but such is not depicted. When backwashing has been completed, the valve in the appropriate backwash water introduction line (for example 169a) is closed. Residual backwash water is removed from the anion exchanger utilizing a pressurized air purge. An air supply line 170 supplies pressurized air for this purpose with communication between line 170 and exchangers 152a, l52b and 1526 being provided respectively by valved lines 172a, l72b and 1726'. Solution leaves the column during backwashing and the subsequent purging via backwash outlet line (for example, line l69b) and is routed to waste disposal.

After backwash water has been so purged, pressurized air flow is stopped by closing the valve in the ,appropriate line 172a, 1721; or 172c and the valve in the appropriate backwash outlet line, for example line 169b, is closed. The anion exchanger being regenerated is then vented to the atmosphere by means of a valve not depicted. Then, regenerating agent isiflowed into the anion exchanger. For this purpose, a regenerating agent supply line 174 is provided, and it communicates respectively with anion exchangers 152a; 15217 and 1526' by means ofvalved lines 176a, 17Gb and 176C. The regenerating agent comprises alkali metal hydroxide, for example, potassium or sodium hydroxide, or mixtures thereof. Preferably, the regenerating agent 3 comprises sodium hydroxide utilized in aqueous solution at a concentration of by volume. The alkali metal hydroxide reacts with'the resin to provide resin in the hydroxide form and releaseanions containing v hexavalent chromium and produce alkali metal salts containing hexavalent chromium in a negative radical,

that is alkali metal chromates and dichromates. The amount of alkali metal hydroxide introduced is based on calculations of how much chromium entered and therefore is in a particular column with a sufficient amount being added so that when solution is removed from the column as described hereafter as the effluent of regeneration as defined hereafter, the efiluent of regeneration will contain from about to about of the hexavalent chromium in the column just prior to initiation of regeneration. When the appropriate amount of regenerating agent has been introduced, flow of such into the column is stopped, and liquid is forced from the column utilizing a pressurized air purge. The pressurized air for the air purge is supplied through line 170. When the air purge has forced the liquid from the column that readily is removed thereby, air introduction is continued and rinse water is simultaneously routed into the column for a predetermined time and is forced therefrom by the pressurized air. Rinse water flow is provided through piping described below. The steams leaving the column both as a result of purging of liquid in the column from addition of regenerating agent and as a result of purging liquid from the rinse water introduction make up the effluent of regeneration. These streams leave the columns 152a, l52b and 1526 respectively through exit lines 178a, 1782; and 1786. Thus, alkali metal hydroxide is introduced to provide an aqueous effluent from the column containing cations and from about 90% to about 95% of the hexavalent chromium present in a column just prior to initiation of regeneration and to leave in the anion exchange column the remainder of the cations and hexavalent chromium. Preferably, the amount of hexavalent chromium removed from the column in the effluentis about 95% of that present just prior to initiation of regeneration.

The exit lines 178a, l78b and 178C communicate re spectively with three-way valves 180a, 180b and 180C. During forcing of effluent of regeneration from a column, the appropriate three-way valve is positioned so as to route liquid into a respective communicating line 182a, 18212 or 1820. The lines 182a, 1821; and 1820 communicate with a main line 184.

After the effluent of regeneration has left the column being regenerated and has been routed to line 184, the pressurized air supply is stopped by closing of the valve in the appropriate line 172a, 1721; or 1720, but water introduction is continued to fill the column and simultaneously with the shutting off of the air supply the appropriate threeway valve 180a, l80b or 1806 is repositioned so as to' connect the respective line 178a, l78b or l78c'into a recirculation loop described below. Piping for this water introduction as well as the rinse water introduction to provide part of the effluent of regeneration comprises a main water line 186 which in turn communicates with valved branch lines 188a, l88b and 188C which respectively communicate with cation ex changer entrance lines 190a, 19Gb and 190C with 188a and 190abeing associated with exchanger 150a, l88b and 190b being associated with exchanger 150b and 188c and 1900 being associated with exchanger 150C. The piping for the water addition system also includes a pipe 192a connecting exchangers 150a and 152a, a pipe l92b connecting exchangers 150b and 152b, a pipe 192c connecting exchangers 150C and 1520; the pipes 192a, 1921) and 192c each contain valves which are not depicted. Thus, in introducing water into anion exchanger152a, the water from main line 186 enters valved line 188a and then line l90a, then passes through cation exchanger 150a, then through line 192a and into anion exchanger 152a to fill the same. The exchangers 15212 and 1520 are filled with water in corresponding fashion when they are in aregeneration cycle.

The passage of the water through the cation exchanger before entry into the anion exchanger provides the advantage of removing cations therefrom including divalent cations such as calcium and magnesium ions so that service water can be utilized without danger of hardness in the water clogging the exchanger system. In other words, the water can be drawn from a source of water containing divalent cations (for example, calcium and magnesium ions) since these cations are removed prior to entry of the water into the anion exchanger.

Once the anion exchange column has been filled with water, water addition is stopped by closing the valve in the appropriate line 188a, 188b, 1880, the appropriate vent valve is closed, and recirculation between the anion exchange column and its associated cation exchange column is started. The recirculation loop including ion exchange columns 150a and 152a consists of line 178a, a line 1940 containing a pump 196a, valve 180a joining lines 178a, 182a and 194a and positioned to provide flow between lines 178a and 194a, line 190a communicating at its upstream end with the downstream end of line 194a, and line 192a. The recirculation loop including ion exchange columns and 1521; consists of line 17812, a line 194b containing a pump 196b, valve 180b joining lines 178b, 182k and 19419 and positioned to provide flow between lines l78b and 194b, line 190b communicating at its upstream end with the downstream end of line 194b, and line 192b. The recirculation loop including ion exchange columns 150C and 1526' consists of line 1786, a line 1946 containing a pump 196e, valve 180a joining lines 178e, 1820 and 194C and positioned to provide flow between lines 1786 and 1946, line 190C communicating at its upstream end with the downstream end of line 1946, and line 192C. To carry out recirculation, the appropriate pump 196a, 19611 or 196C is operated. Recirculation is carried out between the anion exchanger and the cation exchanger in a loop until anions including the hexavalent chromium containing anions in the anion exchanger being treated react with the anion exchange resin in that exchanger and cations react with the resin in the cation exchanger. Conductivity sensors 198a, l98b and 198care provided respectively sensing in lines 1780, 178b and 178C to indicate. when reaction has been completed.

After reaction has been completed, recirculation is stopped by stopping the appropriate pump 196a, 196b or 1966.

Preferably, the regeneration cycle including backwashing, air purging, introduction of regenerating agent, removal of resulting solution, introduction of rinse water and removal of resulting solution introduction of water and recirculation is carried out automatically so that the various valves including the valves in lines 188a, 188b, 1880, 158a, and 158b and 158C, 169a, l69b and the valves 180a, 180b and 1800 are operated automatically in accordance" with a predetermined schedule.

The liquid passing through line 184 comprises an aqueous solution containing cations (alkali metal ions from the regenerating agent for the anion exchange zone) and a concentration of hexavalent chromium in the form of anions substantially higher than the relatively low concentration leaving compartment 98 or in tank 132. It flows into a holding tank 200. Intermittently liquid is pumped from tank 200 by pump 202 through a valved line 204 through a cation exchange column 206 referred to hereafter as cation exchanger 206, exchanger 206 and column 206 (see FIG. 1A). Effluent from the cation exchanger passes into plater solution storage tank 32 via a line 208. When the liquid passes through the cation exchanger 206, cations in the liquid react with a resin in the exchanger and are exchanged for hydrogen ions, and the effluent from the cation exchanger is an aqueous solution of chromic acid containing, for example 10 to 50 grams per liter of chromic acid expressed as CrO that is an acidic aqueous solution comprising a concentration of hexavalent chromium in the form of anions substantially higher than the concentration of .such in the stream leaving compartment 98 through line 126 or in tank 132. Meters 210a and 21012 (pH meters) are provided on either side of exchanger 206 for the easy determination of whether the resin bed in exchanger 206 is depleted, that is whether the resin in the exchanger is spent and needs to be regenerated.

The ion exchange resin utilized in cation exchanger 206 is of the strong acid type and is used in the hydrogen form. It contains sulfonic acid functional groups in a polymer matrix and is prepared, for example by the nuclear sulfonation of styrene-divinylbenzene. The resin utilized is highly resistant to oxidation because the concentration of chromic acid to which the column is exposed is such as to be highly oxidative. The resin contains a relatively high level of cross-linking to provide such oxidation resistance. A suitable resin is sold under the tradename Amberlite 200 by Rohm and Haas; this resin is sold in the sodium form and is converted to the hydrogen form for use. The manufacturer of Amberlite 200 has measured its oxidative resistance in terms of moisture content increase upon immersion of the resin in 3% hydrogen peroxide solution for 72 hours and has found a moisture content increase from 47% before immersion to a moisture content of 55% at the end of the immersion (72-hour) period.

-The solution leaving compartment 98 through line 126 contains, for example, 0.1 to 0.3 grams per liter of chromic acid expressed as CrO and the solution leaving the exchange system through line 208 contains for example 10 to 50 grams per liter of chromic acid expressed as CrO Thus, the ion exchanger system serves to concentrate the chromic acid solution so that it can be treated as described hereafter so as to be suitable for reuse in the plater tanks. I

An evaporation system is provided to control the volume of liquid in the system and to remove water from the system inasmuch as liquid from line 94 comprises chromic acid as a concentration of, for example, to grams per liter of chromic acid expressed as CrO and the liquid from line 208 has a concentration of chromic acid expressed as CrO of, for example, 10 to 50 grams per liter. In other words, the water added via line 76 and the excess water added into the system through line 174 has to be removed to concentrate solution to plating strength. The evaporation system comprises an evaporator 211 defining an evaporation zone where solution is heated against steam passing in indirect heat exchange relation as indicated by lines 212a, and 212b. The evaporator hasa concentrated liquid exit line 214 and a vapor exit line 216. Also communieating with the evaporator is a liquid feed line 218 containing a pump 220 and a valve 222 and communicating at its upstream end with the bottom of plating tank 32. ln utilizing the evaporator, solution from plater storage tank 32 is passed from the plater storage tank to the evaporator and concentrated solution is passed from the evaporator to the plater storage tank. More particularly, the liquid is pumped by pump 220 from the lower portion of plater storage tank 32 through line 218 into the evaporator where it is heated indirectly against steam passing as indicated by lines 212a and 212b and subjected to vacuum whereby water is flashed off leaving through line 216 (such water is condensed and disposed to waste) and concentrated liquid leaves the evaporator via line 214 and is fed back into plater storage tank 32.

A number of inventive concepts are included in the above described process and some of these are explained in detail hereinafter.

One inventive concept is the use of two different eation exchangers in treating an aqueous solution containing hexavalent chromium in the form of anions to produce a relatively concentrated chromic acid solution. In the described system the two different exchangers are one of the exchangers 150a, 150b, 150C and also the exchanger 206. This enables the use of different ion exchange resins for different cation exchange functions. lnother words, this allows the use of a resin which is highly resistant to oxidation in exchanger 206 where the somewhat concentrated stream of hexavalent chromium in the form of anions is present which is a strong oxidizing agent. On the other hand, a resin which is not as highly resistant to oxidation can be utilized in exchangers 150a, 150b and 150C where a relatively low concentration of hexavalent chromium in the form of anions is present which is a relatively mild oxidizing agent. The use of these different resins permit the use ofa resin in exchanger 206 which performs very effectively and at the same time minimizes cost (the highly oxidative resistant resin being substantially more expensive than the less resistant type of resin).

Another inventive concept presented is the provision of recirculation between the plating storage zone defined by tank 32 and the plating zone defined by tanks 12, l4, 16, 18 and succeeding tank 20, passing solution from lines 94 and 208 to the plating storage zone and passing solution from the plating storage zone to an evaporation zone to reduce system volume and provide concentrating effect. This combination of steps maximizes the concentration of the material being treated by evaporation and permits treatment of a relatively large volume of material during evaporation. This minimizes concentration control difficulties which would be inherent in treating the small volume from lines 94 and 208 in an evaporator to regulate the concentration of the large volume of solution in tank 32 and the tanks 12, 14, 16, 18 and 20. Treatment of the streams from lines 208 and 94 in the evaporator without prior addition of these to the large volume of plater solution in the system can also result in temperature control difficulties during evaporation in keeping the temperature of the liquid being treated below its flash point; this problem is eliminated by utilization of the inventive concept of thisparagraph.

The above inventive concepts are illustrated in the following specific example.

EXAMPLE The system of FIGS. 1A and 1B is utilized. The system is generally operated as described above. The specities are presented below.

Steel strip (36 inch width) having been preliminarily treated in electrolytic cleaning and pickling steps is passed through the system. Chrome plating is applied in tanks 12, 14, 16 and 18. Objectionable oxide coating is removed in tank 20. The plated strip is washed in tanks 52 and 54. In tank 56, hexavalent chrome oxide coating is applied. In tank 58 the strip is washed. Finally the strip is further washed in spray washer 96. The strip exiting from washer 96 is ready for drying. The line speed is 1550 feet per minute.

Each of the tanks 12, 14, 16, 18 and 20 contains 1700 gallons of plater solution. The plater solution is an aqueous solution comprising 150 grams per liter of chromic acid expressed as CrO 1 gram per liter of sulfuric acid expressed as S0, and 3grams per liter of metal silicofluorides expressed as SiF One hundred gallons per minute of plater solution enters each tank and 100 gallons per minute of plater solution leaves each tank. Of the 100 gallons per minute entering tank 12, 2 gallons per minute enters through prewet sprays 19. The temperature of the plater solution is 1 15F.

Tank 52 contains 800 gallons of aqueous solution containing about 90 grams per liter of chromic acid expressed as CrO Solution enters and leaves tank 52 at the average rate of about 1.5 gallons per minute.

Tank 54 contains 1700 gallons of aqueous solution containing about 35 grams per liter of chromic acid expressed as CrO Solution enters and leaves tank 54 at the average rate of about 1.5 gallons per minute.

Tank 56 contains 1700 gallons of aqueous solution containing chromic acid at a concentration of about 35 grams per liter expressed as Cr O 0.15 grams per liter of sulfuric acid expressed as SO. and 0.6 grams per liter of metal silicofluorides expressed as SiF Solution passes in and out of tank 56 at a rate of approximately gallons per minute. The temperature of solution in tank 56 is 120F.

Tank 58 contains 800 gallons of aqueous solution. This solution has a concentration of chromic acid expressed as CrO of IO grams per liter. 1.5 gallons per minute of solution continuously pass in and out of this tank.

In washer 96, 20 gallons per minute of demineralized water enters through sprays 1040; 20 gallons per minute overflows from compartment 104 to compartment 102; 20 gallons per minute is sprayed by sprays 102C, 20 gallons overflows from compartment I02 to compartment 100; 20 gallons per minute is sprayed through sprays 1000; 20 gallons per minute overflows from compartment 100 to compartment 98; 20 gallons per minute is sprayed through sprays 98c; and 20 gallons per minute leaves through line 126. The water entering through sprays 104C is at a temperature of F. The use of water at this temperature aids in strip washing and drying.

The solution leaving through line 126 has a concentration of chromic acid expressed as CrO of 0.2 grams per liter. The concentration of chromic acid expressed as CrO in compartment 104 is in low parts per million.

The strip leaving washer 96 contains trace or no amounts of chromic acid.

Tank 132 is a 6,000 gallon tank and controller 138 operates valve 140 to maintain a continuous stream of liquid through pipe 134. This stream of liquid is an aqueous solution having a concentration of hexavalent chromium in the form of anions expressed as CrO of 0.2 grams per liter.

The heat exchanger 142 operates to cool the solution from pipe 134 to 100F.

The solution having been so cooled is routed to two of the three cation exchangers 150a, l50b and 1500. These exchangers and each of the other ion exchangers in the system are 10 feet high and 54 inches in diameter and of the conventional type where the ion exchange resin is maintained upon a screen which is positioned in the bottom of the exchanger, the exchanger is vertically oriented and the solution to be treated enters the top and leaves the bottom. The ion exchange resin utilized in the exchangers 150a, l50b and 1506 is Amberlite lR-l2O Plus, and it is utilized in the hydrogen form.

The effluent from the two cation exchangers (of 150a, lSOb and 1506) that are on stream is passed to the two anion exchangers which communicate with such cation exchangers. The anion exchangers 1520, 152b and 152C contain as an ion exchange resin Amberlite IRA-900. This resin is supplied in the chloride form and is converted to the hydroxide form for use.

The effluent from the anion exchangers is demineralized water which is routed to storage tank 162.

Every eight hours oneset of cation and anion exchangers is taken out of the line and a fresh set of cation and anion exchangers is put in the line. For example, if cation exchangers 150a and 15017 and anion exchangers 152iz and l52b are being utilized and the resin in exchangers 150a and 152a is the most depleted, exchangers 150a and 152a are taken out of the line and exchangers'150c and 1526 are put in the line.

The cation exchanger taken out of the line is regenerated prior to regenerating the associated anion exchanger. The regeneration comprises backwashing, then treating with a regenerating agent consisting of 10% aqueous sulfuric acid and then rinsing. Flow rates and times are in accordance with the resin manufacturers recommendations.

The anion exchanger which is taken out of the line is regenerated first by backwashing for 10 minutes utilizing service water having cations removed therefrom introduced through the appropriate backwash water introduction line. When the 10-minute time period has ended, the valve in the backwash water line is closed automatically; then the appropriate valve in the air purge line (that is, the valve in the appropriate branch line 172a, l72b or 1726) is automatically opened and the residual backwash water is forced from the anion exchanger through the appropriate backwash water outlet line. After the valve for air purging has been open for 10 minutes, it automatically closes and the anion exchanger is automatically vented to the atmosphere.

Then the valve in the appropriate regenerating agent introduction line 176a, l76b or 1760 automatically .opens. The regenerating agent utilized is aqueous sodium hydroxide solution containing 10% sodium hydroxide by volume. Four hundred gallons of regenerating agent is introduced over a 40-minute time period. At this point, the valve in the regenerating agent introduction line automatically closes, and the valve in the appropriate line 172a, l72b or 1720 automatically opens and the appropriate three-way valve 180a, 1801; or 1.806 is automatically positioned to route liquid to tank 200. After 10 minutes, the air has forced substantially all of the liquid from the column being regenerated. At this point the valves in the appropriate lines 188a, 188b, 1880, 192a, l92b, 192C automatically open whereby water passes through a cation exchanger and then into the anion exchanger which is being regenerated. The water is introduced at the rate of 60 gallons per minute. After 1 minute, the appropriate valve is closed to shut off the air. The 60 gallons of water introduced before the air is shut off serves to rinse out residual solution heavily laden with chrome, and carry it to tank 200. The 460 gallons routed to tank 200 contain of the hexavalent chromium which was in the resin in the column just prior to initiation of regeneration.

When the air is shut off, the appropriate threeway valve 180a, [80b or 1806 is automatically repositioned to recirculation position. (that is, to provide communication, for example between lines 178a and 194a) and water introduction is continued to fill the column. Then water introduction is automatically stopped by means of a conductivity switch, the vent valve closes and the appropriate pump 1960, 196b or 1966 is automatically started and recirculation between the anion exchanger being regenerated and its associated cation exchanger is carried out. This recirculation is continued until the conductivity cell 198a, l98b or 198C registers 25 mi cromhos thereby indicating reaction of the sodium chromates and dichromates with the resins to establish a level of chromate and dichromate in the anion exchange resin.

Except during regeneration of column 206, solution is pumped by pump 202 from tank 200 through exchanger 206. The exchange resin in this exchanger is Amberlite 200 which has been converted to the hydrogen form; it is much more highly oxidative resistant than the cation exchange resin Amberlite IR-l20 Plus utilized in the cation exchangers a, l50b and 150C. Passage of the solution through the cation exchange resin removes cations and the effluent from the cation exchanger 206 is aqueous chromic acid solution containing 25 grams of chromic acid per liter expressed as CrO The differential pH as indicated by meters 210a and 210b indicates resin bed depletion, and a predetermined higher pH being registered by meter 210a indicates that the resin bed is ready to be regenerated. Regeneration is carried out utilizing backwashing, introduction of regenerating agent and rinsing in accordance with the manufacturers directions for the particular resin being utilized. The regenerating agent utilized is 10% by volume aqueous sulfuric acid.

Tank 32 contains about 6,000 gallons when the line tanks are full.

Recirculation is carried out between tank 32 and evaporator 211 with 70 gallons per minute being pumped by pump 220 whereby the concentration of solution in tank 32 is maintained at 150 grams per liter of chromic acid expressed as CrO Steam is introduced through line 212a to supply heat. A vacuum of 26 inches of Hg is utilized in evaporator 211; this vacuum is provided by an eductor on line 216.

The recirculation through lines 44 and 46 between heat exchanger 42 and tank 32 is at the rate of 500 gallons per minute and cooling water is passed through exchanger 42 via lines 50a and 50b countercurrent to the flow of plater solution to remove heat from the system and maintain the temperature of the plater solution at 1 F.

The use of different cation exchangers for the cation exchange upstream of the anion exchange and for the cation exchange downstream of the anion exchange allows the use of different cation exchange resins so that a highly oxidative resistant resin can be utilized in ex changer 206 where such resin is desirable because of the relatively high concentration of hexavalent chromium in the form of anions while a resin which is not highly oxidative resistant but which is less expensive can be utilized in exchangers 150a, l50b and 1506 where the concentration of hexavalent chromium in the form of anions is lower and less oxidation resistance is needed. Thus, the use of different cation exchangers and different resins significantly contributes to the economy of the system. The removal of only 95% of the hexavalent chromium from the anion exchangers 152a, l52b and 1520 during regeneration maximizes the concentration of chromic acid in the effluent of regenera tion thereby significantly contributing further to the economy of the system by reducing the load on the evaporation system. Finally, the recirculation between the plater tanks and tank 32 in combination with adding the streams from lines 94 and 208 into tank 32 and treating solution from tank 32 in the evaporator provides significantly more accurate plater solution volume and concentration control than if streams 94 and 208 were passed directly to an evaporator and has the further advantage of allowing continuous evaporator operation on a relatively large volume of liquid compared to the volume of the streams from 94 and 208 thereby permitting continuous evaporator operation without danger of exceeding the flash point of the solution being treated.

- The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. For example, inventive concepts herein are useful in treating not only chrome plated steel strip but also other products wettcd with chromic acid solution. Therefore, in view of the variations that are readily understood to come within the limits of the invention, such limits are defined by the scope of the appended claims.

We claim:

1. Method of producing an aqueous solution having a concentration of hexavalent chromium in the form of anions (expressed as CrO ranging from 10 to 50 grams per liter from an aqueous solution comprising a concentration of hexavalent chromium in the form of anions (expressed as CrO on the order of 0.1 to 0.3 grams per liter and cations selected from the group consisting of iron ions and trivalent chromium ions, said method comprising a. treating the solution comprising a concentration of hexavalent chromium in the form of anions (expressed as CrO on the order of 0.1 to 0.3 grams per liter and cations selected from the group consisting of iron ions and trivalent chromium ions in a first cation exchange zone with cation exchange resin of the strong acid type and in the hydrogen form to replace cations with hydrogen ions and produce an aqueous effluent containing the hexavalent chromium in the form of anions and hydro gen ions;

b. treating the effluent from step (a) in an anion exchange zone with strong base anion exchange resin of the quaternary ammonium type in the hydroxide form to react the hexavalent chromium in the form of anions in effluent of step (a) with the resin;

0. regenerating the resin in the anion exchange zone using a regenerating agent comprising alkali metal hydroxide to produce an aqueous effluent comprising alkali metal cations originating from the alkali metal hydroxide and a concentration of hexavalent chromium in the form of anions such as to provide the concentration in the effluent produced in step d. treating the effluent of step (c) in a second cation exchange zone with cation exchange resin of the strong acid type in the hydrogen form to replace cations with hydrogen ions and produce an effluent which is an acidic aqueous solution comprising a concentration of hexavalent chromium in the form of anions (expressed as CF03) ranging from 10 to 50 grams per liter;

e. the cation exchange resin of step (a) being oxidative resistant under the conditions of step (a) but not under the conditions of step (d), and the cation exchange resin of step (d) being oxidative resistant under the conditions of step (d), whereby economy in resin cost is achieved.

2. Method as recited in claim 1 in which in step (d) the cation exchange resin is highly cross-linked. 

1. METHOD OF PRODUCING AN AQUEOUS SOLUTION HAVING A CONCENTRATION OF HEXAVALENT CHROMIUM IN THE FORM OF ANIONS (EXPRESSED AS CRO3) RANGING FROM 10 TO 50 GRAMS PER LITER FROM AN AQUEOUS SOLUTION COMPRISING A CENTRATION OF HEXAVALENT CHROMIUM IN THE FORM OF ANIONS (EXPRESSED AS CRO3) ON THE ORDER OF 0.1 TO 0.3 GRAMS PER LITER AND CATIONS SELECTED FROM THE GROUP COMPRISING OF IRON IONS AND TRIVALENT CHROMIUM IONS, SAID METHOD COMPRISING A. TREATING THE SOLUTION COMPRISING A CONCENTRATION OF HEXAVALENT CHROIUMIN THE FORM OF ANIONS (EXPRESSED AS CRO3) ON THE ORDER OF 0.1 TO 0.3 GRAMS PER LITER AND CAUTIONS SELECTED FROM THE GROUP CONSISTING OF IRONS IONS AND TRIVALENT CHROIUM IONS IN A FIRST CATION EXCHANGE ZONE, WITH CATION EXCHANGE RESIN OF THE STRONG ACID TYPE AND IN THE HYDROGEN FORM TO REPLACE CATIONS WITH HYDROGEN IONS AND PRODUCE AN AQUEOUS EFFLUENT CONTAINING THE HEXAVALENT CHROMIUM IN THE FORM OF ANIONS AND HYDROGEN IONS, B. TREATING THE EFFLUENT FROM STEP (A) IN AN ANION EXCHANGE ZONE WITH STRONG BASE ANION EXCHANGE RESIN OF THE QUATERNARY AMMONIUM TYPE IN THE HYDROXIDE FORM TO REACT THE HEXAVALENT CHROMIUM IN THE FORM OF ANIONS IN EFFLUENT OF STEP (A) WITH THE RESIN, C. REGENERATING THE RESIN IN THE ANION EXCHANGE ZONE USING A REGENERATING AGENT COMPRISING ALKALI METAL PRODUCE AN AQUEOUS EFFLUENT COMPRISING ALKALI METAL CATIONS ORGINATING FROM THE ALKALI METAL HYDROXIDE AND A CONCENTRATION OF HEXAVALENT CHROMIUM IN THE FORM OF ANIONS SUCH AS TO PROVIDE THE CONCENTRATION IN THE EFFLUENT PRODUCED IN STEP (D), D. TREATING THE EFFLUENT OF STEP (C) IN A SECOND CATION EXCHANGE ZONE WITH CATION EXCHANGE RESIN OF THE STRONG ACID TYPE IN THE HYDROGEN FORM TO REPLACE CATIONS WITH HYDROGEN IONS AND PRODUCE AN EFFLUENT WHICH IS AN ACIDIC AQUCOUS SOLUTION COMPRISING A CONCENTRATION OF HEXAVALENT CHROMIUM IN THE FORM OF ANIONS (EXPRESSED AS CRO3) RANGING FROM 10 TO 50 GRAMS PERLITER, C. THE CATION EXCHAMGE RESIN OF STEP (A) BEING OXIDATIVE RESISTANT UNDER THE CONDITIONS OF STEP (A) BUT NOT UNDER THE CONDITIONS OF STEP (D), AND THE CATION EXCHANGE RESIN OF STEP (D) BEING OXIDATIVE RESITANT UNDER THE CONDITIONS OF STEP (D), WHEREBY ECONOMY IN RESIN COST IS ACHIVED.
 2. Method as recited in claim 1 in which in step (d) the cation exchange resin is highly cross-linked. 